When did it start to go downhill to unwatchable?

There's gems here and there. I find that they often get better after a few viewings. I even thought the show was getting weak in the mid 90s but there are still gems.

I noticed things were going downhill near season 10. They always showed the episodes out of sync here in the UK so it's hard to completely pinpoint when this happened, but I noticed more episodes around this time that I didn't find as funny, and the plots seemed more wacky with random celebrity appearances that didn't make much sense. In terms of being unwatchable, I bailed out long before that happened, so I can't answer that.